Invasive Species in Native and Non-native Ranges

Armenia, ECRC/AUAUSA, University of Montana, University of CaliforniaArgentina, Universidad Nacional de La PambaTurkey, Adnan Menderes UniversityGeorgia, Institute of BotanyRomania, Institute Of Biological ResearchHungary, Institute of Ecology and Botany

General view of the experimental field in Armenia

Studying germination in the native and non-native range of a species can provide unique insights into processes of range expansion and adaptation; however, traits related to germination have rarely been compared between native and nonnative populations. In a series of common garden experiments, we explored whether differences in the seasonality of precipitation, specifically, summer drought vs summer rain, and the amount and variation of annual and seasonal precipitation affect the germination responses of populations of an annual ruderal plant, Centaurea solstitialis, from its native range and from two non-native regions with different climates. We found that seeds from all native populations, irrespective of the precipitation seasonality of the region in which they occurred, and non-native populations from regions with dry summers displayed similarly high germination proportions and rates. In contrast, genotypes from the non-native region with predominantly summer rain exhibited much lower germination fractions and rates and ecology. Organisms transported by humans to regions where they are not native (exotics) commonly face novel selective forces, which given enough genetic variation, may trigger novel evolutionary responses. The worldwide distribution of this species encompasses environments with contrasting precipitation regimes within both native and non-native ranges. Specifically, some of the regions where C. solstitialis grows are characterized by a Mediterranean-type climate with wet winters and dry summers, whereas other regions have a precipitation regime in which most of the precipitation falls during the summer, and winters are substantially drier. In all regions, the species germinates primarily in autumn (Sheley and Larson 1994, Hierro et al. 2006, L. Khetsuriani, L. Janoian, and K. Andonian unpubl.); thus, winter conditions may affect its survival. Here, by conducting a series of common garden experiments in a growth chamber, we investigated whether contrasting differences in the seasonality of precipitation and changes in surrogates for environmental quality (e.g. precipitation totals) and risk (e.g. inter-annual variation in precipitation) affect germination responses of C. solstitialis populations occurring across its native range and in two climatically distinct non-native regions.

To investigate the potential effects of seasonality of precipitation on C. solstitialis germination, we conducted three successive seed collections from populations occurring in regions exepte France, Crete and Armenia, where seeds were pooled within populations. Mean cumulative germination percentages (91 SE) of pappus and non-pappus seeds of C. solstitialis populations plotted against the coefficient of variation of winter precipitation and the probability of occurring a good winter.

Pappus seeds maintained a strong association with variation in winter precipitation (r__0.91, pB0.001), but this relationship did not hold for nonpappus seeds (r__0.36, p_0.172). In addition, germination of both pappus and non-pappus seeds were no longer correlated with the probability of good winters (r_0.44, p_0.117 and r_0.11, p_0.387, respectively). Without Argentina, the association between germination proportions of pappus seeds and variation in annual precipitation improved slightly (r__0.70, p_0.017), whereas the correlation of these proportions with the probability of good years remained non-significant (r_0.39, p_0.150). Finally, as before, germination fractions of non-pappus seeds were not correlated with any of the measures of annual risk, and germination percentages of both seed morphs were not associated with any of the measures describing environmental quality (p_0.250 in all cases).

Clines in these studies corresponded to variation in general climatic patterns, such as changes in climate between northern and southern latitudes (Maron et al. 2004, 2007) or between coastal versus inland environments . In contrast to these results, our comparisons based on general climatic patterns (i.e. summer drought vs summer rain) did not detect parallel clines in germination traits for populations from native and non-native ranges, as all native populations, irrespective of the climate in which they occurred, and non-native populations from the region with a summer-drought climate displayed similarly high germination proportions and rates; whereas non-native genotypes from the region with a summer rain regime exhibited much lower germination fractions and rates. On the other hand, our comparisons based on precipitation variables, which are commonly used as surrogates for environmental quality and risk, showed that for the most abundant seed morph, seeds with a pappus, germination responses of populations in both native and non-native ranges correlated strongly with ‘risk’ experienced during the winter. Specifically, and as predicted by bethedging theory, germination fractions of pappus seeds were lower in native and non-native populations experiencing greater inter-annual variation in winter precipitation (Fig. 4). For non-pappus seeds, however, this correlation was greatly influenced by non-native genotypes from central Argentina, which are from the region with the highest variation in winter precipitation of all the studied regions and exhibited the lowest proportions of germinating seeds in all our experiments (Fig. 2_4); after removing central Argentina from analyses, there was no association between germination fractions of non-pappus seeds and winter precipitation variation. Similarly, germination fractions of both pappus and non-pappus seeds correlated with probability of occurrence of good winters only in the presence of Argentinean variables. Overall, these findings suggest that rather than general climatic patterns, the degree of risk experienced at early developmental stages could exert an important control over the germination strategy of C. solstitialis populations in both native and non-native ranges. In addition, they reveal the largely unique nature among studied populations of seed germination in nonnative genotypes from central Argentina. Germination fractions of pappus seeds were also correlated with variation in annual precipitation, suggesting that overall annual risk could also play a role in the germination behavior of C. solstitialis populations. Indeed, populations experiencing comparable variation in winter precipitation in the native and non-native range tended to display similar germination fractions for this seed type . In contrast, for non-pappus seeds the link between degree of dormancy and level of winter risk does not hold when outlier Argentinean variables are removed from analyses, providing weaker support for bet-hedging across C. solstitialis populations.

Several mechanisms could be responsible for the genetic differentiation in germination traits of Californian versus Argentinean populations, including coincidental introductions, genetic drift, and natural selection operating on phenotypes formed by either a novel combination of genes or pre-adapted genotypes (i.e. the sorting-out hypothesis _ Mu¨ller-Scha¨rer and Steinger 2004; see Leger and ice 2007 for a comprehensive discussion on these mechanisms). Outcrossing plants partition most of their genetic diversity within, rather than among, populations, which increases the probability of possessing high genetic variation upon introduction because even a few immigrants can carry much of the species’ genetic variation.

Full article available in Oikos 118: 529_538, 2009

doi: 10.1111/j.1600-0706.2009.17283.x,

# 2009 The Authors. Journal compilation # 2009 Oikos

Subject Editor: Pia Mutikainen. Accepted 31 October 2008

“Germination responses of an invasive species in native and non-native ranges”